Non-Positive Displacement Hydraulic Pumps

They are also known as hydro-dynamic pumps. In these pumps the pressure produced, is proportional to the rotor speed. In other words, the fluid is displaced and transferred using the inertia of the fluid in motion. These pumps are incapable of withstanding high pressures and are generally used for low-pressure and high-volume flow applications. Normally their maximum pressure capacity is limited to 20-30 kgf/cm3. They are primarily used for transporting fluids from one location to the other and find little use in the hydraulic or fluid power industry.

Because of fewer numbers of moving parts, non-positive displacement pumps cost less and operate with little maintenance. They make use of Newton’s first law of motion to move the fluid against the system resistance. Although these pumps provide a smooth and continuous flow, their flow output is reduced as the system resistance (resistance to flow) is increased. In fact it is possible to completely block the outlet to stop all flow even while the pump is running at the designed speed. Thus the pump flow rate depends not only on the rotational speed (rpm) at which it is driven but also on the resistance of the external system. As the resistance of the external system increases, some of the fluid will slip back, causing a reduction in the discharge flow rate. When the resistance of the external system becomes very large, the pump will produce no flow and thus its volumetric efficiency becomes zero. Examples of these pumps are the centrifugal and axial (propeller) pumps.

In a centrifugal pump, a simple sketch of which is illustrated in Figure 3.2, rotational inertia is imparted to the fluid. Centrifugal pumps are not self-priming and must be positioned below the fluid level.


Principle of operation

The fluid from the inlet port enters at the center of the impeller. The rotating impeller imparts centrifugal force to the fluid and causes it to move radially outward. This results in the fluid being forced through the outlet discharge port of the housing. The tips of the impeller blades merely move through the fluid while the rotational speed maintains the fluid pressure corresponding to the centrifugal force established.

Centrifugal pumps are generally used in pumping stations, for delivering water to homes and factories. The advantages of non-positive displacement pumps are:

• Low initial cost and minimum maintenance
• Simplicity of operation and high reliability
• Capable of handling any type of fluid, for example sludge and slurries.

Since the impeller imparts kinetic energy to the fluid, centrifugal pumps are also known as hydrokinetic power generators.

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